Variation Of Barrier Height Research Articles

A Two Dimensional Tunneling Resistance Transmission Line Model for Nanoscale Parallel Electrical Contacts

Contact resistance and current crowding are important to nanoscale electrical contacts. In this paper, we present a self-consistent model to characterize partially overlapped parallel contacts with varying specific contact resistivity along the contact length. For parallel tunneling contacts formed between contacting members separated by a thin insulating gap, we examine the local voltage-dependent variation of potential barrier height and tunneling current along the contact length, by solving the lumped circuit transmission line model (TLM) equations coupled with the tunneling current self consistently. The current and voltage distribution along the parallel tunneling contacts and their overall contact resistance are analyzed in detail, for various input voltage, electrical contact dimension, and material properties (i.e. work function, sheet resistance of the contact members, and permittivity of the insulating layer). It is found the existing one-dimensional (1D) tunneling junction models become less reliable when the tunneling layer thickness becomes smaller or the applied voltage becomes larger. In these regimes, the proposed self-consistent model may provide a more accurate evaluation of the parallel tunneling contacts. For the special case of constant ohmic specific contact resistivity along the contact length, our theory has been spot-checked with finite element method (FEM) based numerical simulations. This work provides insights on the design, and potential engineering, of nanoscale electrical contacts with controlled current distribution and contact resistance via engineered spatially varying contact layer properties and geometry.

Barriers to internal rotation in methylimidazole isomers determined by rotational spectroscopy.

The rotational spectra of N-, 2-, 4-, and 5-methylimidazole are reported and analyzed. Liquid N-methylimidazole was vaporized from a reservoir, and each of 2-, 4-, and 5-methylimidazole was laser-vaporized from a solid target prior to mixing with argon buffer gas and undergoing supersonic expansion from a pulsed nozzle. The spectra were recorded by chirped-pulse Fourier transform microwave spectroscopy in the 7.0-18.5 GHz frequency range. Rotational constants, A0, B0, and C0, centrifugal distortion constants, DJ, DJK, DK, d1, and d2, and nuclear quadrupole coupling constants of nitrogen atoms, χaa(N1), χbb(N1) - χcc(N1), χaa(N3), and χbb(N3) - χcc(N3), are determined from experimentally measured transition frequencies. Data recorded for isotopologues containing 13C or 15N are used to determine the rs coordinates of all heavy atoms in N-, 2-, and 4-methylimidazole. The results allow fitting of parameters in the Hamiltonian that describes internal rotation of the CH3 group about its local C3 axis. The V3 terms in the periodic potential energy functions that describe the internal rotation in N-, 2-, 4-, and 5-methylimidazole are determined to be 185.104(11), 122.7529(38), 317.20(14), and 386.001(19) cm-1, respectively, by the internal axis method. The experiments are supported by density functional theory calculations. Observed variations in barrier height are explained with reference to the symmetry of overlap between a π-like orbital on the CH3 group and π-orbitals on the aromatic ring.

Two-Dimensional/Three-Dimensional Schottky Junction Photovoltaic Devices Realized by the Direct CVD Growth of vdW 2D PtSe2 Layers on Silicon.

Two-dimensional (2D) platinum diselenide (PtSe2) layers are a new class of near-atom-thick 2D crystals in a van der Waals-assembled structure similar to previously explored many other 2D transition-metal dichalcogenides (2D TMDs). They exhibit distinct advantages over conventional 2D TMDs for electronics and optoelectronics applications such as metallic-to-semiconducting transition, decently high carrier mobility, and low growth temperature. Despite such superiority, much of their electrical properties have remained mostly unexplored, leaving their full technological potential far from being realized. Herein, we report 2D/three-dimensional Schottky junction devices based on vertically aligned metallic 2D PtSe2 layers integrated on Si wafers. We directly grew 2D PtSe2 layers of controlled orientation and carrier transport characteristics via a low-temperature chemical vapor deposition process and investigated 2D PtSe2/Si Schottky junction properties. We unveiled a comprehensive set of material parameters, which decisively confirm the presence of excellent Schottky junctions, i.e., high-current rectification, small ideality factor, and temperature-dependent variation of Schottky barrier heights. Moreover, we observed strong photovoltaic effects in the 2D PtSe2/Si Schottky junction devices and extended them to realize flexible photovoltaic devices. This study is believed to significantly broaden the versatility of 2D PtSe2 layers in practical and futuristic electronic devices.

An In-Depth Study on Electrical and Hydrogen Sensing Characteristics of ZnO Thin Film With Radio Frequency Sputtered Gold Schottky Contacts

The electrical and hydrogen sensing characteristics of radio frequency sputtered Au/ZnO thin film Schottky diodes on n-silicon substrate have been investigated over a wide temperature range. The Current-voltage characterizations of the device in the temperature range of 25 °C to 200 °C confirm its excellent rectifying property with forward to reverse current ratio of 1610 at an external bias of 5 V. The ideality factor in the range of 4.12 to 2.98 is obtained for Au/ZnO Schottky diode in the aforementioned temperature range, at atmospheric conditions. On exposing diode to hydrogen, a reduction in the ideality factor is observed which makes thermionic emission more prominent. The proposed device has proven to be hydrogen sensitive, on account of the lateral shift observed in ${I}$ – ${V}$ characteristics at different hydrogen concentrations (50 ppm–1000 ppm). The maximum barrier height variation of 99 meV and sensitivity of 144% have been observed at 1000 ppm hydrogen at 200 °C. A detailed perusal of the steady-state reaction kinetics of the sensor using ${I}$ – ${V}$ characteristics affirmed that the atomistic hydrogen adsorption at Au/ZnO interface is accountable for the barrier height modulation. The studied sensor depicts a remarkable performance for the high-temperature detection.

Contact resistance at graphene/MoS2 lateral heterostructures

The contact resistance at two-dimensional graphene/MoS2 lateral heterojunctions is theoretically studied, using first-principles simulations based on density functional theory and the nonequilibrium Green's function method. The computed contact resistance lies in the range of 102 to 104 Ω μm, depending on the contact edge symmetry (armchair or zigzag) and termination (Mo and/or S terminated). This large variation in the contact resistance arises from the variation in the interface barrier height, which is sensitive to the presence of polar C-Mo bonds or sulfur dangling bonds at the interface. These results highlight that the control of the edge symmetry and/or edge termination is crucial to achieve a low contact resistance (in the range of a few hundred ohms micrometer) at graphene/MoS2 lateral heterojunctions for 2D material-based field-effect devices.

Dual Liquid Junction Photoelectrochemistry: Part II. Open-Circuit Photovoltage Variations Due to Surface Chemistry, Interfacial Dipoles, and Non-Ohmic Junctions at Back Contacts

Dual-liquid-junction photoelectrochemistry and finite-element computational modeling quantified the effect on open-circuit photovoltage, Voc, of varying barrier heights at the back, traditionally ohmic contact to a semiconductor. Variations in experimental back-contact barrier heights included changes in the redox potential energy of the contacting phase afforded by a series of nonaqueous, metallocene-based redox couples that demonstrate facile, one-electron transfer and dipole-based band edge shifts due changes in the chemical species at the semiconductor surface. Variation in semiconductor surface chemistry included hydrogen-terminated Si(111) as well as methyl-terminated Si(111) that yields a shift in band-edge alignment of ∼0.3 eV relative to hydrogen termination. While methylation of n-Si improves Voc values at rectifying contacts, methylation at an ohmic contact has a deleterious impact on Voc values. We discuss the present experimental and computational results in the context of non-ideal semiconductor contacts.

Physical, photochemical, and extended piezoelectric studies of orthorhombic ZnSnN2 nanocolumn arrays

This study reports the piezo-related properties of ZnSnN2 (ZTN) fabricated through natural Sn3N4 and Zn thickness gradients deposited oppositely on a fluorine-doped tin oxide substrate to form Zn–Sn3N4 composition spreads to enhance the relative variation of the cation ratios and to promote the formation of orthorhombic ZTN. No moving shutter was incorporated into combinatorial magnetron sputtering to minimize fabrication complexity. The Sn3N4-rich deposition (Location 1) of the Zn–Sn3N4 composition spread exhibited Sn3N4 and the single crystallinity of orthorhombic (Pna21) ZTN nanocolumn arrays, which grew along the [0 0 1] direction, confirmed by locked-coupled X-ray diffraction and transmission electron microscopy. The constituent element diffusion and variation of the atomic binding state of constituent elements along the substrate normal were tracked by depth profiling using secondary ion mass spectroscopy and X-ray photoelectron spectroscopy. The band gap of ZTN (approximately 2.0 eV) was estimated from a UV–vis spectrum. The piezotronic and piezophototronic effects of ZTN were determined through a facile current–voltage measurement, which were explained by the Schottky barrier height variations. Piezophotocatalysis exhibited the highest rate constant (k, approximately 9 × 10−3/min) of the measurements, which was attributed to the reduced recombination of the photogenerated e−–h+ pairs because of the piezopotential distribution. Additionally, O2− radicals were predominately indicated in the degradation process.

Manipulating the magnetism and resistance state of Mn:ZnO/Pb(Zr0.52Ti0.48)O3 heterostructured films through electric fields

Mn:ZnO/Pb(Zr0.52Ti0.48)O3 (PZT) heterostructured films have been prepared on Pt/Ti/SiO2/Si wafers by a sol-gel process. Nonvolatile and reversible manipulation of the magnetism and resistance by electric fields has been realized. Compared with the saturation magnetic moment (Ms) in the +3.0 V case, the modulation gain of Ms can reach 270% in the −3.0 V case at room temperature. The resistance change is attributed to the interfacial potential barrier height variation and the formation of an accumulation (or depletion) layer at the Mn:ZnO/PZT interface, which can be regulated by the ferroelectric polarization direction. The magnetism of Mn:ZnO originates from bound magnetic polarons. The mobile carrier variation in Mn:ZnO, owing to interfacial polarization coupling and the ferroelectric field effect, enables the electric manipulation of the magnetism in the Mn:ZnO/PZT heterostructured films. This work presents an effective method for modulating the magnetism of magnetic semiconductors and provides a promising avenue for multifunctional devices with both electric and magnetic functionalities.

Origin of threshold voltage fluctuation caused by ion implantation to source and drain extensions of silicon-on-insulator triple-gate fin-type field-effect transistors using three-dimensional process and device simulations

The threshold voltage (Vth) fluctuation induced by ion implantation (I/I) in the source and drain extensions (SDEs) of a silicon-on-insulator (SOI) triple-gate (Tri-Gate) fin-type field-effect transistor (FinFET) was analyzed by both three-dimensional (3D) process and device simulations collaboratively. The origin of the Vth fluctuation induced by the SDE I/I is basically a variation of a bottleneck barrier height (BBH) due to implanted arsenic (As+) ions. In particular, a very low and broad Vth distribution in the saturation region is due to percolative conduction in addition to the BBH variation. Moreover, it is surprisingly found that the Vth fluctuation is mostly characterized by the BBH of only a top surface center line of a Si fin of the device. Our collaborative approach by 3D process and device simulations is dispensable for the accurate investigation of variability-tolerant devices. The obtained results are beneficial for the research and development of such future devices.

Constrained Chemical Dynamics of CO Dissociation/Hydrogenation on Rh Surfaces.

Among noble metal catalysts, rhodium (Rh) is unique in its ability to perform a one-step synthesis of ethanol from syngas. The first steps following the adsorption of syngas on Rh surfaces are assumed to be responsible for the conversion of CO and the selectivity effects between C1 , C2 , and oxygenated species. In the current work, constrained ab initio molecular dynamics are applied to investigate the kinetics of CO dissociation and hydrogenation over flat and stepped Rh surfaces. The obtained barriers for the Rh(111) surface are in good agreement with the literature data. On the stepped Rh(211) surface, a large site-dependent variation in barrier height is shown, with the upper terrace exhibiting behavior comparable to the Rh(111) surface, whereas the barriers over the lower terrace site are generally significantly lower. The rate constants are calculated using transition state theory for both surfaces, and are applied successfully in a microkinetic model, confirming the predicted impact on CO conversion and CH4 /C1 -oxygenate/C2 Hn selectivity. In addition to the high-accuracy energetics and rate constants reported for CO dissociation/hydrogenation and the presentation of an updated microkinetic mechanism for Rh catalysts, the applicability of constrained molecular dynamics for reaction barrier calculation is confirmed, and sensitive pathways affecting the selectivity between formaldehyde/methanol over Rh catalysts are highlighted.

Study of TMR with Different Ferromagnetic Material and Variations in Spin-Split, Thickness and Oxide Barrier Height of a MTJ Memory Device

Study of TMR with Different Ferromagnetic Material and Variations in Spin-Split, Thickness and Oxide Barrier Height of a MTJ Memory Device

Partial reset HDR image sensor with improved fixed pattern noise performance

An efficient method is presented to achieve 4T pixel dynamic range extension whilst keeping the fixed pattern noise (FPN) low. The method utilizes the floating diffusion node as a secondary photocharge integration node with a partial reset applied to the floating diffusion node. The output image signal is a recombination of the linear signal integrated in the photodiode node, and a compressed signal integrated in the floating diffusion node. The recombination algorithm suppresses the FPN stemming from the variable transfer gate potential barrier height. An image sensor array of 1344x1008 pixels was built in 0.11um technology. We demonstrate the preservation of low light sensitivity of 4T pixels, a dark noise of 3.5e-, and a dynamic range for the current sensor of 115 dB, with potential to greatly exceed 120-dB. Further, the FPN stemming from the transfer gate potential barrier height variation does not degrade the image; the pixel compressive response can be controlled on a per frame basis depending on the scene, and the HDR scene is captured during one frame. The sensor combines the properties that are crucial for automotive and other machine vision applications.

A comparative study of Hf and Ta incorporations in the dielectric of Pd-WO3-SiC Schottky-diode hydrogen sensor

An investigation on the incorporation of two different kinds of high-κ dielectrics (HfO2 and Ta2O5) in the dielectric of Pd-WO3-SiC Schottky diode is presented. It is found that while the surface morphology of the WO3 and WHfO films is almost the same, the WTaO film has the smoothest surface due to suppression of oxygen vacancies in WO3 by the Ta incorporation, as supported by XPS analysis. The current-voltage characteristics are examined under a wide range of temperature and hydrogen concentration. Upon exposure to 10,000 ppm H2/air, the diodes based on WHfO and WTaO show a maximum hydrogen response of 89 and 147 respectively, both higher than that (31) of the control sample with WO3. From the kinetics analysis, it is demonstrated that more hydrogen atoms are accumulated at the Pd/WHfO and Pd/WTaO interfaces than their Pd/WO3 counterpart due to larger enthalpy change for hydrogen adsorption on passivated surface, resulting in a greater barrier-height variation at the interface and thus better sensing performance for the two devices with ternary oxide.

Characterization of Ag Schottky Barriers on Be0.02Mg0.26ZnO/ZnO Heterostructures

The BeMgZnO/ZnO heterostructures are capable of producing sufficiently high sheet electron densities to allow field effect transistor operation near or at the LO phonon plasmon resonance frequency for minimal LO phonon lifetimes and high electron velocity. Schottky barriers are imperative for the implementation of the aforementioned devices. Therefore, we have undertaken fabrication and characterization of Ag Schottky barriers on Zn‐polar Be0.02Mg0.26ZnO/ZnO heterostructures, exhibiting the said two‐dimensional electron gas (2DEG), grown by molecular beam epitaxy. Ag Schottky barriers are characterized by current–voltage (I–V) measurements in the temperature range from 80 to 400 K. At room temperature, the highest barrier height of 1.07 eV and an ideality factor of 1.22 are achieved with a rectification ratio of about eight orders of magnitude. Richardson constants of 38 ± 22 and 29 ± 20 A cm−2 K−2 are found using modified Richardson plots from Schottky diodes fabricated on two different structures. These values are consistent with the theoretical estimate of 36 A cm−2 K−2 for Be0.02Mg0.26ZnO. The temperature variation of barrier heights and ideality factors, an aberration from pure thermionic behavior, has been explained with plausible spatial inhomogeneity of barrier height with three Gaussian distributions in the temperature ranges of 80–200, 220–280, and 300–400 K.

Spontaneous fission and competing ground state decay modes of actinide and transactinide nuclei

Based on the preformed cluster model, we have carried out a comprehensive theoretical study on the decay paths of ground state actinide and transactinide nuclei, specifically from $^{232}\mathrm{U}$ to $^{264}\mathrm{Hs}$ exhibiting the phenomenon of spontaneous fission (SF). This is an extension of our earlier studies on $\ensuremath{\alpha}$ decay, exotic cluster emission, and heavy particle radioactivity, where an effort is made to identify the most probable fragments in the SF process. These observations in turn, could provide a testing ground for future SF half-life measurements. To obtain a clear picture of the dynamics involved, the variations of fragmentation potential, preformation factor, and decay barrier height have been examined. The calculated potential energy surfaces show a change from a predominantly asymmetric fission to a symmetric fragmentation with the increase in the N/Z ratio of parent nuclei. In addition, an exclusive analysis of SF with $\ensuremath{\alpha}$ and other possible cluster emissions for the $^{232,234,236,238}\mathrm{U}$ parents was made to have better insight of nuclear structure information. In other words, the comparative nature of $\ensuremath{\alpha}$, cluster, heavy fragment, and SF decay paths is analyzed in view of shell closure property of the decay fragments. Interestingly, the calculated decay half-lives for the $^{82}\mathrm{Ge}$ heavy cluster are in fact shown to lie within the limits of experiments, thereby presenting themselves as exciting new possibilities which may be validated via future experiments.

Piezo-tunnel effect in Al/Al2O3/Al junctions elaborated by atomic layer deposition

In this work, the electrical transport in Al/Al2O3/Al junctions under mechanical stress is investigated in the perspective to use them as strain sensors. The metal/insulator/metal junctions are elaborated with a low temperature process (≤200 °C) fully compatible with CMOS back-end-of-line. The conduction mechanism in the structure is found to be Fowler-Nordheim tunneling, and efforts are made to extract the relevant physical parameters. Gauge factors up to −32.5 were found in the fabricated devices under tensile stress. Finally, theoretical mechanical considerations give strong evidence that strain sensitivity in Al/Al2O3/Al structures originates not only from geometrical deformations but also from the variation of interface barrier height and/or effective electronic mass in the tunneling oxide layer.

Piezotronic and Piezophototronic Properties of Orthorhombic ZnSnN2 Fabricated Using Zn–Sn3N4 Composition Spreads through Combinatorial Reactive Sputtering

This paper presents a combinatorial methodology for fabricating orthorhombic ZnSnN2 (ZTN) by using Zn–Sn3N4 composition spreads. This study is the first to verify the substantial piezotronic and piezophototronic features of ZTN on the basis of asymmetric current–voltage (I–V) characteristics. Regarding the piezophototronic effect, at a −5 V bias, the current density was enhanced up to 2.5 times when the applied pressure increased from 0.625 to 2.5 GPa. Schottky barrier height variations at S1 and S2 were calculated under a pressure of 2.5 GPa and were observed to have increased and decreased by approximately 1.0 and 22 mV, respectively. The results clarified the I–V behavior and also supported the proposed energy-band structure evolution of piezotronic and piezophototronic ZTN. In addition, ZTN formation was verified through X-ray photoelectron spectroscopy and X-ray diffraction. A deconvolution algorithm was employed to validate the ratio of orthorhombic ZTN (Pna21) (approximately 30%). In addition, UV–v...

Structural and Electrical Properties of Ag/n-TiO2/p-Si/Al Heterostructure Fabricated by Pulsed Laser Deposition Technique

We have investigated the structural and electrical characteristics of the Ag/n-TiO2/p-Si/Al heterostructure. Thin films of pure TiO2 were deposited on p-type silicon (100) by optimized pulsed laser ablation with a KrF-excimer laser in an oxygen-controlled environment. X-ray diffraction analysis showed the formation of crystalline TiO2 film having a tetragonal texture with a strong (210) plane as the preferred direction. High purity aluminium and silver metals were deposited to obtain ohmic contacts on p-Si and n-TiO2, respectively. The current–voltage (I–V) characteristics of the fabricated heterostructure were studied by using thermionic emission diffusion mechanism over the temperature range of 80–300 K. Parameters such as barrier height and ideality factor were derived from the measured I–V data of the heterostructure. The detailed analysis of I–V measurements revealed good rectifying behavior in the inhomogeneous Ag/n-TiO2/p-Si(100)/Al heterostructure. The variations of barrier height and ideality factor with temperature and the non-linearity of the activation energy plot confirmed that barrier heights at the interface follow Gaussian distributions. The value of Richardson’s constant was found to be 6.73 × 105 Am−2 K−2, which is of the order of the theoretical value 3.2 × 105 Am−2 K−2. The capacitance–voltage (C–V) measurements of the heterostructure were investigated as a function of temperature. The frequency dependence (Mott–Schottky plot) of the C–V characteristics was also studied. These measurements indicate the occurrence of a built-in barrier and impurity concentration in TiO2 film. The optical studies were also performed using a UV–Vis spectrophotometer. The optical band gap energy of TiO2 films was found to be 3.60 eV.

Analysis of photoconductive mechanisms of organic-on-inorganic photodiodes

In this work, it is shown that choosing an organic-on-inorganic Schottky diode for photoconductive sensing by a using a power law exponent (PLE or γ) determined at a single bias point is a limited approach. The standard literature approach does not highlight any bias voltage effects on the distribution of interface state density and other operationally important parameters. In this paper we suggest a new empirical method that holistically highlights the variation of γ with voltage, irradiance and temperature to reach a more informed choice of photosensor for real applications. We obtain a simple, plausible relation of the variation of barrier height, Φ, with voltage, irradiance and temperature. The method is evaluated with data collected previously for Schottky diodes of structure Al/p-Si/organic-semiconductor (OSC)/Au, where OSC is Coumarin-doped with graphene oxide (GO), Cobalt Phthacyanine (CoPC) doped with GO or PCBM doped with GO, respectively. The method reproduces published data for the three diodes reported at specific bias and provides for the first time some qualitative evidence of barrier height variation with light intensity, for which a possible physical basis is also given. Typically, Schottky barrier height is characterized using dark current leading to an under reporting of the effect of illumination on barrier height. Finally, since recombination mechanisms are gauged on the basis of the magnitude of PLE, the method facilitates the identification of the recombination mechanism at a given bias.

Topological Insulator Bi2Se3/Si-Nanowire-Based p-n Junction Diode for High-Performance Near-Infrared Photodetector.

Chemically derived topological insulator Bi2Se3 nanoflake/Si nanowire (SiNWs) heterojunctions were fabricated employing all eco-friendly cost-effective chemical route for the first time. X-ray diffraction studies confirmed proper phase formation of Bi2Se3 nanoflakes. The morphological features of the individual components and time-evolved hybrid structures were studied using field emission scanning electron microscope. High resolution transmission electron microscopic studies were performed to investigate the actual nature of junction whereas elemental distributions at junction, along with overall stoichiometry of the samples were analyzed using energy dispersive X-ray studies. Temperature dependent current-voltage characteristics and variation of barrier height and ideality factor was studied between 50 and 300 K. An increase in barrier height and decrease in the ideality factor were observed with increasing temperature for the sample. The rectification ratio (I+/I-) for SiNWs substrate over pristine Si substrate under dark and near-infrared (NIR) irradiation of 890 nm was found to be 3.63 and 10.44, respectively. Furthermore, opto-electrical characterizations were performed for different light power intensities and highest photo responsivity and detectivity were determined to be 934.1 A/W and 2.30 × 1013 Jones, respectively. Those values are appreciably higher than previous reports for topological insulator based devices. Thus, this work establishes a hybrid system based on topological insulator Bi2Se3 nanoflake and Si nanowire as the newest efficient candidate for advanced optoelectronic materials.